Radio location techniques that utilize a signal's Time Difference of Arrival (TDOA) have been widely used in many mobile transmitter location systems. The technique utilizes the propagation delay of the radio signal to infer the geographic distance. Three or more propagation delay measurements allow location computation. TDOA techniques can be applied to either the uplink signals received by the base stations from the mobile or the downlink signals transmitted from the base stations received by the mobiles. The use of uplink signals for location computation is commonly referred to as Time of Arrival (TOA). For the TOA case the mobile originates a signal having a single clock reference (the mobile's clock) and the base stations must measure the receive time using a sufficiently accurate multiple clocks. The use of downlink signals is commonly referred to as Enhanced Observed Time Difference (OTD). For the OTD case the base stations must have a sufficiently accurate known start time and the mobile provides the receive time measured against a single clock reference. Three or more base stations are used for either the TOA or OTD methods.
Both the TOA and OTD methods share an aspect of geometry having a three to one ratio as well as one end of the measurement being at known location(s). For the TOA method, a signal originating from a single mobile is measured by three or more geographically known measurement locations. For the OTD method, signals originating from three or more geographically dispersed signal sources (base stations at known locations) are measured by a single receiver (mobile). Due to the single end aspect of one end of the measurement combined with the known location(s) at one end of the measurement, for the purpose of location computation, it is simply an algebraic manipulation as to whether the propagation delay measurements are considered as three or more individual propagation delay numbers or two or more time-differences.
The end of the measurement at the geographically disbursed locations (e.g., base stations) require the utilization of a reference clock ("internal clock") with a rate sufficiently accurate to limit the amount of error introduced by any time drift. Rather than synchronize all locations to a common clock reference such as GPS or to use highly stable clocks at all locations, it is accepted practice to allow timing errors at the base stations. The errors are measured by a receiver device. This Real Time Difference (RTD) receiver reports the time differences referenced to a sufficiently stable internal clock. The TOA or OTD data and the RTD data is sent to a single computation location. These measurement and data collection processes (TOA/OTD data vs. RTD data and location computation) are asynchronous and may be made at different points in time. When TOA or OTD data measured at time t.sub.1 are used with RTD data measured at time t.sub.2, the actual RTD at t.sub.1 is not known and the computation is in error by that amount. For accurate location calculations, there is a need to provide a method whereby this error can be removed while allowing the measurement processes to remain asynchronous.